1. Field of the Invention
This invention pertains generally to circuit interrupters and, more particularly, to circuit breakers including a bimetal in series with separable contacts. The invention also pertains to methods for determining bimetal temperature and/or bimetal resistance.
2. Background Information
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
In certain circuit breaker applications (e.g., without limitation, arc fault detection), the voltage across the thermal bimetal element is employed as an indirect measurement of the circuit breaker load current. Sensing current with a bimetal element is complicated by the variation of that element's impedance as a function of temperature. This variation results in inaccuracies in the measurement of the amplitude of the measured current. For example, the bimetal element's impedance can vary as much as about 70% with temperature over the normal operating range of the circuit breaker depending upon the type of bimetallic material used.
As is typical with most metals, the bimetal impedance has a positive temperature coefficient (PTC). In other words, resistance increases with temperature. If the design of the circuit breaker electronics assumes that the bimetal resistance is constant, then any resistance-temperature variation of the bimetal can introduce error in the current sensing. This variation can cause proportional inaccuracies in the measurement of the load current, thereby affecting performance of control algorithms implemented in the circuit breaker electronics. As bimetal temperature rises, the voltage drop across the bimetal increases for a given amount of load current. The net effect is that the load current appears larger than it really is. Depending on the control algorithm, the potential result could be an errant command to trip the circuit breaker.
U.S. Pat. No. 4,486,733 discloses an electrothermal responsive protective mechanism of the bimetal type having also an ambient temperature compensating bimetal element to maintain accurate operation of the protective mechanism despite ambient temperature changes.
U.S. Pat. No. 6,813,131 discloses a circuit breaker trip assembly including a bimetal element and a thermistor. The thermistor is a temperature-sensing device, which is adapted to respond to the temperature of the bimetal element, in order to measure the approximate temperature thereof. For example, if located in close proximity to the bimetal element, the thermistor will be subjected to less extreme temperatures since the power dissipated by the bimetal element is dispersed to the cooler ambient environment within the circuit breaker housing. Even in close proximity, the temperature sensed by the thermistor proximate the bimetal element, although less extreme, is still proportional to the actual temperature of the bimetal element. An amplifier input is referenced to the bimetal element output terminal. The bimetal impedance has a positive temperature coefficient, while the thermistor has a negative temperature coefficient. Responsive to the thermistor, the amplifier provides a negative gain with respect to the bimetal voltage. In turn, the trip assembly provides a trip signal as a function of the compensated amplifier output voltage.
There is room for improvement in circuit breakers including a bimetal in series with separable contacts.
There is also room for improvement in methods for determining bimetal temperature and/or bimetal resistance.